Method for the production of alicyclic polycarboxylic acid esters from partial esters of aromatic polycarboxylic acids

ABSTRACT

The invention relates to a process for preparing full esters of alicyclic polycarboxylic acids from partial esters of aromatic polycarboxylic acids by hydrogenation of the partial ester followed by esterification of this compound.

The invention relates to a process for preparing full esters ofalicyclic polycarboxylic acids from partial esters of aromaticpolycarboxylic acids (by hydrogenation followed by esterification).

Cycloaliphatic polycarboxylic esters, such as the esters orcyclohexane-1,2-dicarboxylic acid, are used as a component inlubricating oils and as an auxiliary in metalworking. They are also usedas plasticizers for polyolefins.

For plasticizing PVC, use is frequently made of esters of phthalic acid,for example the dibutyl, dioctyl, dinonyl, or didecyl ester. Since thesephthalates are increasingly regarded as hazardous to health, it islikely that their use in plastics could be restricted. Cycloaliphaticpolycarboxylic esters, some of which have been described in theliterature as plasticizers for plastics, could then be available asreplacements, even though their performance profile is somewhatdifferent.

Alicyclic polycarboxylic esters may be obtained by esterifying thecorresponding alicyclic polycarboxylic acids, or their anhydrides, orpartial esters. Few of these compounds are available at low cost, sincethey have to be prepared in multistage syntheses.

For example, cyclohexane-1,2-dicarboxylic esters can be obtained viaDiels-Alder reaction of butadiene with maleic anhydride, or withfumarates or maleics, followed by hydrogenation of the olefinic doublebond in the cyclohexane derivative. If maleic anhydride is used asstarting material, the synthesis also includes an esterification.

In many cases, alicyclic polycarboxylic esters are prepared byring-hydrogenation of the corresponding aromatic polycarboxylic esters.

U.S. Pat. Nos. 5,286,898 and 5,319,129 describe processes which canhydrogenate dimethyl terephthalate on supported Pd catalysts treatedwith Ni, Pt and/or Ru, to give the corresponding dimethylhexahydroterephthalate at temperatures of 140° C. or above and at apressure of from 50 to 170 bar. DE 28 23 165 discloses a process for thehydrogenation of aromatic carboxylic esters on supported Ni, Ru, Rh,and/or Pd catalysts, to give the corresponding cycloaliphatic carboxylicesters, at from 70 to 250° C. and from 30 to 200 bar. U.S. Pat. No.3,027,398 describes the hydrogenation of dimethyl terephthalate onsupported Ru catalysts at from 110 to 140° C. and from 35 to 105 bar.

DE 197 56 913 and WO 99/32427 disclose a process for the hydrogenationof benzene polycarboxylic esters to give the correspondingcycloaliphatic compounds. Here, use is made of supported catalysts whichcomprise Ru on its own or together with at least one metal of the1^(st), 7^(th) or 8^(th) transition group of the periodic table, andhave 50% of macropores.

The aromatic carboxylic esters used for the ring hydrogenation processare mostly prepared by esterifying the corresponding carboxylic acids ortheir anhydrides, the corresponding partial esters being produced as anintermediate in the reaction mixture. To reach full esterification here,relatively long reaction times and/or relatively drastic reactionconditions may be needed.

The processes described in the abovementioned literature for thepreparation of cycloaliphatic polycarboxylic esters are therefore basedon the following sequence of reactions:

-   1. Preparation of an aromatic polycarboxylic ester, a partial ester    generally being formed first and reacted to give the full ester-   2. Purification and work-up of the resultant polycarboxylic ester to    give the pure product-   3. Hydrogenation of the aromatic polycarboxylic ester to give the    corresponding cycloaliphatic polycarboxylic ester-   4. Optionally: purification and work-up of the resultant    cycloaliphatic polycarboxylic ester to give the pure product.

There are also conceivable versions of the process in which unpurifiedcrude ester is hydrogenated. However, a disadvantage of these processesis that the activity of the hydrogenation catalyst is reduced bydeposits deriving from the esterification catalyst, the result being arequirement for frequent catalyst change and a resultant fall incost-effectiveness.

Since catalysis of the esterification reaction is generally homogeneous,the crude product of the esterification has to be freed from catalysts,by-products, and alcohol prior to its hydrogenation, i.e. has to beworked up. This work-up is complicated, since the commonly usedprocesses have two purification stages, and there can therefore be twoyield losses.

An object is therefore to provide a simpler and more cost-effectiveprocess for the preparation of alicyclic polycarboxylic esters.

Surprisingly, it is possible to obtain cycloaliphatic polycarboxylicesters in a simple and cost-effective manner by hydrogenation of anaromatic partial ester of an aromatic polycarboxylic acid or of amixture which comprises one or more partial esters of one or morearomatic polycarboxylic acids, followed by esterification of theresultant cycloaliphatic polycarboxylic partial ester.

The present invention therefore provides a process for the preparationof cycloaliphatic polycarboxylic esters, which comprises

-   a) hydrogenating a partial ester of the corresponding aromatic    polycarboxylic acid or of the corresponding aromatic polycarboxylic    anhydride, and-   b) reacting the resultant cycloaliphatic partial ester with an    alcohol to give the full ester.

The full esters may optionally be purified by filtration, steamdistillation, or stripping with steam.

Starting materials which may be used in the process of the invention arepure substances, e.g. compounds which are isomerically pure with respectto the ester side chain, or else mixtures of isomers, or indeed mixturesof different esters, both with respect to the chain length of the estergroup and with respect to the aromatic system.

One way in which mixtures of isomers with respect to the ester sidechain are produced is, for example, during the preparation of phthalateesters of isononanol, which is a C₉ isomer mixture. It is also possibleto use a mixture of C₈ and C₉ phthalate partial esters and/or anisomeric mixture of 1,2- and 1,4-phthalate partial esters. Partialesters of phthalic acid are its monoesters with one remaining carboxylicacid function.

In one particular embodiment of the process of the invention, a partialester is first prepared from an aromatic polycarboxylic acid or thecorresponding anhydride and an alcohol or alcohol mixture. Processes foresterification of carboxylic acids or their anhydrides are known to theskilled worker. In the esterification of polycarboxylic acids, and inparticular of their anhydrides, partial esters are first produced, i.e.the esterification reaction does not proceed as far as the fullyesterified polycarboxylic ester. For example, in the esterificationreaction of phthalic anhydride with alcohols, the correspondingmonoester is first formed at an elevated temperature by an autocatalyticmechanism. However, if the corresponding diester is desired it isadvantageous to carry out the second esterification stage at a highertemperature and/or with addition of a catalyst.

For the purposes of the present invention, partial esters are compoundswhich, besides at least one ester function, also contain at least onefree carboxylic acid function or anhydride function. One advantage ofthe process of the invention is that in both steps of the process it ispossible to exert influence on the cis/trans ratio of the alicyclicpolycarboxylic acid derivatives, so that products with different transcontents can be obtained. Products obtained by the process of theinvention can have a higher trans content than those obtained in directhydrogenation of the corresponding aromatic polycarboxylic ester. Cisand trans compounds perform differently. The use of trans-rich productsas plasticizers for plastics is advisable if, for example, theplasticizer is required to have relatively low volatility.

A further advantage is that only one purification stage, which may becomposed of two or more substeps, is needed to prepare the alicyclicpolycarboxylic ester by the process of the invention.

The catalysts used in the process of the invention for the hydrogenationof the partial esters are in most cases the same as those known from theliterature for the hydrogenation of aromatic full esters to give thecorresponding cycloaliphatic full esters.

These are mostly supported catalysts. The active metal present in themmay in principle be any of the metals of the 8^(th) group of theperiodic table. The active metals present in them are preferablyplatinum, rhodium, palladium, cobalt, nickel or ruthenium, or a mixtureof two or more of these. Other active metals may be elements of thefirst or seventh transition group of the periodic table, for examplecopper, rhenium, or a combination of these.

The support materials for the hydrogenation catalysts may be: activatedcarbon, aluminum oxide, alumosilicate, silicon dioxide, titaniumdioxide, zirconium dioxide, magnesium oxide, zinc oxide, or a mixture ofthese.

Examples of a material which may be used in the process of the inventionare ruthenium/aluminum oxide catalysts of types H14163 or B4168/10r fromDegussa AG, Dusseldorf, Germany.

The catalysts used in the process of the invention for the hydrogenationof the aromatic partial esters may be prepared by applying at least onemetal of the 8^(th) transition group of the periodic table to a suitablesupport.

The application may be achieved by saturating the support in aqueousmetal salt solutions, by spraying appropriate metal salt solutions ontothe support, or by other suitable methods. Suitable salts of metals ofthe 8^(th) transition group of the periodic table are the nitrates,nitrosyl nitrates, halides, carbonates, carboxylates, acetylacetonates,chloro complexes, nitrito complexes, or amine complexes of thecorresponding metals.

The supports coated or saturated with the metal salt solution are thendried, preferably at temperatures of from 100 to 150° C., and calcinedif desired at temperatures of 200 to 600° C., preferably from 350 to450° C.

The coated and dried and, if desired, calcined supports are thenactivated by treatment with a stream of gas which comprises freehydrogen, and at temperatures from 30 to 600° C., preferably from 100 to450° C. The stream of gas is preferably composed of a mixture ofhydrogen and nitrogen. It can be advantageous to increase the hydrogencontent during the course of the activation. For example, the hydrogencontent in the gas mixture may be 10% at the start of the activation andover 90% at the end of the activation process.

Where appropriate, this activation may be carried out in the samereactor in which the partial ester is hydrogenated. The activation ofthe catalyst may optionally be undertaken in the presence of a liquidphase which trickles over the catalyst.

The amount of the metal salt solution or metal solutions applied to thesupport is such that the total content of active metal, based in eachcase on the total weight of the catalyst, is from 0.01 to 30% by weight,preferably from 0.01 to 5% by weight, very particularly from 0.05 to 2%by weight.

The surface area of metal on the catalysts is from 1 to 50 m²/g. Thesurface area of the metal is determined by the chemisorption methoddescribed by J. Lemaitre et al. “characterization of HeterogeneousCatalysts”, Ed. Francis Delanney, Marcel Dekker, New York 1984, pp.310-324.

The support materials for the catalysts may be macroporous, mesoporous,or microporous, or have pores, various numbers of which fall into thethree ranges mentioned. Their BET surface area is from 5 to 600 m²/g.

The hydrogenation in the process of the invention is preferably carriedout in the liquid phase. The hydrogenation may be carried outcontinuously or batchwise on suspended catalysts or on particulatecatalysts in a fixed bed. In the process of the invention, preference isgiven to continuous hydrogenation on a fixed-bed arrangement ofcatalysts where the products/starting material phase is primarily in theliquid state under the conditions of the reaction.

Various versions of the process of the invention may be selected. It maybe carried out adiabatically or practically isothermally, i.e. with atemperature rise typically smaller than 10° C., in one or more stages.In the latter case, all of the reactors, advantageously tubularreactors, may be operated adiabatically or practically isothermally, orelse one or more may be operated adiabatically and the otherspractically isothermally. It is moreover possible to hydrogenate thearomatic polyesters in a straight pass or with product return.

The process of the invention is carried out in the liquid/gas mixedphase or liquid phase in cocurrent mode in three-phase reactors, thehydrogenating gas being distributed in the liquid startingmaterial/product stream in a manner known per se. In the interests ofuniform liquid distribution, of improved dissipation of the heat ofreaction, and of high space-time yield, the reactors are preferablyoperated with high liquid flow rates of 15 to 120, in particular from 25to 80, m³ per m² of cross section of the empty reactor per hour. If areactor is operated with a straight pass, the liquid hourly spacevelocity (LHSV) over the catalyst may be from 0.1 to 10 h⁻¹.

The hydrogenation may be carried out in the absence, or preferably inthe presence, of a solvent. Solvents which may be used are any of theliquids which form a homogenous solution with the starting material andproduct, have inert behavior under hydrogenation conditions, and areeasy to remove from the product. The solvent may also be a mixture oftwo or more substances and, where appropriate, comprise water.

Examples of substances which may be used as solvents are the following:Straight-chain or cyclic esters, such as tetrahydrofuran or dioxane, andalso aliphatic alcohols whose alkyl radical has from 1 to 13 carbonatoms.

Examples of alcohols which may preferably be used are isopropanol,n-butanol, isobutanol, n-pentanol, 2-ethylhexanol, nonanols, industrialnanonaol mixtures, decanol, and industrial decanol mixtures.

If alcohol is used as solvent it can be advantageous to use the alcoholor alcohol mixture which would be produced during saponification of theproduct (e.g. isononanol as solvent in the hydrogenation of monoisononylphthalate). This prevents any by-product formation viatransesterification. Another preferred solvent is the hydrogenationproduct itself.

By using a solvent it is possible to limit the concentration of aromaticcompounds in the reactor feed, and the result can be better temperaturecontrol achieved in the reactor. This can minimize side-reactions andtherefore increase the yield of product. The content of aromaticcompounds in the reactor feed is preferably from 1 to 35%, in particularfrom 5 to 25%. The desired concentration range can be adjusted via thecirculation rate (quantitative ratio of returned hydrogenation dischargeto starting material) in the case of reactors operated in loop mode.

The hydrogenation in the process of the invention is carried out in thepressure range from 5 to 300 bar, in particular from 15 to 220 bar, veryparticularly from 50 to 200 bar. The hydrogenation temperatures are from50 to 200° C., in particular from 80 to 160° C.

Hydrogenation gases which may be used are any desiredhydrogen-containing gas mixtures in which there are no detrimentalamounts present of catalyst poisons, such as carbon monoxide or hydrogensulfide. Examples of the inert gas constituents are nitrogen andmethane. It is preferable to use hydrogen at a purity greater than 95%,in particular greater than 98%.

The aromatic polycarboxylic acids and, respectively, their correspondinganhydrides preferably used in the process of the invention contain 2, 3or 4 carboxyl functions. Examples of compounds of this type arephthalate acid and trimellitic acid.

The aromatic polycarboxylic acids and, respectively, the correspondingpolycarboxylic anhydrides preferably contain from 1 to 5 fused benzenerings or other annellated radicals. Examples of fused-on benzene ringsare benzene systems, biphenyl systems, naphthalene systems, anthracenesystems, and phenanthrene systems.

Particularly preferred partial esters of the aromatic polycarboxylicacid are the monoesters of 1,2-, 1,3- or 1,4-phthalate acid, (or theiranhydride), and the mono- or di-esters of 1,2,3-, 1,2,4-, or1,3,5-trimellitic acid.

It is preferable for a monoisononyl phthalate or a monooctyl phthalateor a monodecyl phthalate to be reacted in the process of the inventionto give the corresponding cyclohexanedicarboxylic esters.

The process of the invention can use partial esters of the followingaromatic acids:

-   naphthalene-1,2-dicarboxylic acid, naphthalene-1,3-dicarboxylic    acid, naphthalene-1,4-dicarboxylic acid,    naphthalene-1,5-dicarboxylic acid, naphthalene-1,6-dicarboxylic    acid, naphthalene-1,7-dicarboxylic acid,    naphthalene-1,8-dicarboxylic acid, phthalic acid, isophthalic acid,    terephthalic acid, benzene-1,2,3-tricarboxylic acid,    benzene-1,2,4-tricarboxylic acid (trimellitic acid),    benzene-1,3,5-tricarboxylic acid (trimesinic acid),    benzene-1,2,3,4-tetracarboxylic acid,    benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid). It is also    possible to use acids produced from the acids mentioned through    substitution of one or more hydrogen atoms on the aromatic ring by    alkyl, cycloalkyl, or alkoxyalkyl groups. It is also possible to use    polycarboxylic acids having an anthracene skeleton, phenanthrene    skeleton, or diphenyl oxide skeleton.

The alcohol components of the partial and full esters may be branched orunbranched alkyl, cycloalkyl, or alkoxyalkyl groups having from 1 to 25,in particular from 3 to 15, very particularly from 8 to 13, carbonatoms.

It is possible to use alcohol mixtures and, respectively, partial estermixtures with various alcohols. Mixtures may be either alcohols ofdifferent chain length or isomeric mixtures of alcohols of the samechain length.

The process of the invention can also use mixtures which comprise atleast two partial esters. Examples of starting materials of this typemay be: Mixtures produced from tribasic carboxylic acids by partialesterification. Monoalkyl and dialkyl esters may be present alongsideone another in these mixtures.

A mixture with at least two partial esters is produced during partialesterification of a polycarboxylic acid with an alcohol mixture.

Another way in which mixtures of partial esters can be produced is viareaction of a mixture of at least two polycarboxylic acids with analcohol or alcohol mixture.

Low-cost alcohol mixtures are frequently used for esterification inindustry.

Examples of these are

C₅ alcohol mixtures, prepared from linear butenes by hydroformylationfollowed by hydrogenation;

C₅ alcohol mixtures prepared from butene mixtures which comprise linearbutenes and isobutene, by hydroformylation followed by hydrogenation;

C₆ alcohol mixtures prepared from a pentene or from a mixture of two ormore pentenes, by hydroformylation followed by hydrogenation;

C₇ alcohol mixtures prepared from triethylene or dipropene or from ahexane isomer, or from some other mixture of hexane isomers, byhydroformylation followed by hydrogenation;

2-ethylhexanol (2 isomers), prepared by aldol condensation ofn-butyraldehyde followed by hydrogenation;

C₉ alcohol mixtures prepared from C₄ olefins by dimerization,hydroformylation and hydrogenation. The starting materials here may beisobutene or a mixture of linear butenes, or mixtures with linearbutenes and isobutene. The C₄ olefins may be dimerized with the aid ofvarious catalysts, such as protonic acids, zeolites, organometallicnickel compounds, or solid nickel-containing catalysts. The C₈ olefinmixtures may be hydroformylated with the aid of rhodium catalysts orcobalt catalysts. There is therefore a wide variety of industrial C₉alcohol mixtures. C₁₀ alcohol mixtures prepared by tripropylene, byhydroformylation followed by hydrogenation;

2-propylheptanol (2 isomers), prepared by aldol condensation ofvaleraldehyde followed by hydrogenation;

C₁₀ alcohol mixtures prepared from a mixture of at least two C₅aldehydes by aldol condensation followed by hydrogenation;

C₁₃ alcohol mixtures prepared from hexaethylene, tetrapropylene, ortributene, by hydroformylation followed by hydrogenation.

Other alcohol mixtures may be obtained by hydroformylation followed byhydrogenation from olefins or olefin mixtures which are produced, forexample, in Fischer-Tropsch syntheses, in dehydrogenation processes onhydrocarbons, in metathesis reactions, in the polygas process, or inother industrial processes.

It is moreover possible to use olefin mixtures with olefins of varying Cnumbers for preparing alcohol mixtures.

The process of the invention can use any of the partial ester mixturesprepared from aromatic polycarboxylic acids and the above-mentionedalcohol mixtures. According to the invention, preference is given toesters prepared from phthalic acid or phthalic anhydride and from analcohol or mixture of isomeric alcohols having from 1 to 25 carbonatoms.

The partial esters used may be in the form of isomerically pure estersor of a mixture of isomeric partial esters, or of a mixture of variousnon-isomeric partial esters, or preferably of a reaction mixtureproduced during preparation of partial esters, where appropriate aftercatalyst removal.

The partial esters used in the process of the invention may be puresubstances or industrial mixtures, e.g. from the esterificationreaction. Examples of compounds present in mixtures of this type,besides the partial ester, are, for example, full esters, polycarboxylicacid/polycarboxylic anhydride (in each case from 0 to 15 mol %, based onthe partial ester), alcohol, water, and/or the esterification catalyst.

Aromatic polycarboxylic acids or their anhydrides and alcohols (alcoholmixtures) may optionally be fed into the hydrogenation stage in theintended ratio, separately or together. For example, phthalic anhydrideand alcohol (alcohol mixture) may be fed into the outer loop of ahydrogenation reactor operated in loop mode. Partial ester formationoccurs here primarily in the outer loop.

The partial esterification of the aromatic polycarboxylic acids may becarried out by known processes without catalysis or with catalysis byLewis or Brönstedt acids or metal compounds.

Preparation of partial esters from anhydrides of aromatic polycarboxylicacids generally needs no catalyst. For example, phthalic anhydridereacts with alcohols in the temperature range from 110 to 160° C. veryquickly to give the corresponding partial esters. The reaction of ananhydride of an aromatic carboxylic acid may be carried out with anexcess of alcohol. Very small amounts of full ester may be present inthe partial ester mixture. Since the hydrogenation-discharge is in allcases esterified, it is advantageous for the amount of excess alcoholused in the partial ester preparation to be already that intended forthe esterification stage which follows the hydrogenation.

Besides the alicyclic partial ester(s), the product-discharges from thehydrogenation may comprise full esters and alicyclic polycarboxylicacids, alcohol, and also solvents, by-products, and water.

These mixtures are preferably reacted to give the full esters by methodsknown per se, without further purification. This post-esterification maytake place by an autocatalytic or catalytic mechanism, for example usingBrönstedt or Lewis acids. Quite irrespective of the type of catalysisselected, the result is always a temperature-dependent equilibriumbetween the starting materials (partial ester and, where appropriate,carboxylic acid and alcohol) and the products (esters and water). Inorder to shift the equilibrium in favor of the full ester, use may bemade of entrainer with the aid of which the water of the reaction can beremoved from the mixture. Since the alcohols used for the esterificationhave lower boiling points than the polycarboxylic acid, and its partialesters, and its full esters, and are not fully miscible with water, theyare used as entrainer, which can be returned to the process after waterremoval.

The alcohol or alcohol mixture used to form the full ester andsimultaneously serving as entrainer is used in excess, this preferablybeing from 5 to 50%, in particular from 10 to 30%, of the amount neededto form the full ester from the polycarboxylic acid.

Esterification catalysts which may be used are acids, such as sulfuricacid, methanesulfonic acid, or p-toluenesulfonic acid, or metals orcompounds of these. Examples of those suitable are tin, titanium,zirconium, which may be used as finely divided metals or advantageouslyin the form of their salts, or of the oxides or, of the soluble organiccompounds. Unlike protic acids, the metal catalysts are high-temperaturecatalysts which achieve their full activity only at temperatures above180° C. However, their use is preferred since the amount of by-productswhich they form, for example olefins from the alcohol used, is smallerthan with protic catalysis. Examples of metal catalysts are tin powder,tin (II) oxide, tin (II) oxalate, titanium esters, such astetraisopropyl orthotitanate or tetrabutyl orthotitanate, and alsozirconium esters, such as tetrabutyl zirconate.

The alcohol or alcohol mixture used during the full esterification ofthe cycloaliphatic partial ester may be the same as that which would beproduced or is produced during saponification of the partial ester ofthe aromatic polycarboxylic acid.

In one preferred embodiment of the invention, the partial ester is firstprepared, e.g. as in the patent literature cited.

The alcohol or alcohol mixture used in the full esterification in stepb) of the process of the invention may then be the same as that whichwas used to prepare the partial ester.

It is also possible for another alcohol or another alcohol mixture to beused during the esterification in step b) of the process of theinvention. For example, it is possible to hydrogenate a partial esterhaving a C₈ alcohol component and to react this compound with a C₉alcohol component to give the full ester.

The catalyst concentration depends on the nature of the catalyst. Forthe titanium compounds whose use is preferred it is from 0.005 to 1.0%by weight based on the reaction mixture, in particular from 0.01 to 0.5%by weight, very particularly from 0.01 to 0.1% by weight.

When titanium catalysts are used, their reaction temperatures are from160 to 270° C., preferably from 180 to 250° C. The ideal temperaturesdepend on the starting materials, the progress of the reaction, and thecatalyst concentration. They may easily be determined by trials for eachparticular case. Higher temperatures increase the reaction rates andfavor side-reactions, such as elimination of water from alcohols or theformation of colored by-products. For removal of the water of thereaction it is useful that the alcohol can be removed from the reactionmixture by distillation. The desired temperature or the desiredtemperature range may be set via the pressure in the reaction vessel. Inthe case of low-boiling alcohols, therefore, the reaction is carried outat superatmospheric pressure, and in the case of higher-boiling alcoholsit is carried out at subatmospheric pressure. For example, the operatingtemperature range during the reaction of phthalic anhydride with amixture of isomeric nonanoles is from 170 to 250° C. in the pressurerange from 1 bar to 10 m bar.

The liquid to be returned to the reaction may be composed to some extentor entirely of alcohol, which is obtained by work-up of the azeotropicdistillate. It is also possible for the work-up to be carried out at alater juncture and for the liquid removed to be replaced entirely or tosome extent by fresh alcohol, i.e. from alcohol provided in a feedvessel.

The crude ester mixtures, which besides the full ester(s) comprisealcohol, very small amounts of partial ester, catalyst or its downstreamproducts, and, where appropriate, by-products, are worked up by methodsknown per se. This work-up encompasses the following steps: removal ofthe excess alcohol and, where appropriate, low boilers, optionallyincluding steam distillation, neutralization of the acids present,conversion of the catalyst into a residue capable of easy filtration,removal of the solids, and, where appropriate, drying. The sequence ofthese steps can differ, depending on the work-up methods used.

Work-up methods for crude ester mixtures are described by way of examplein DE 197 21 347 C2.

The present invention further provides the use of the alicyclicpolycarboxylic esters prepared according to the invention asplasticizers in plastics. Preferred plastics are PVC, and homo- andcopolymers based on ethylene, on propylene, on vinyl acetate, onglycidyl acrylate, on glycidyl methacrylate, on acrylates, or onacrylates having branched or unbranched alkyl radicals bonded to theoxygen atom of the ester group and having from one to ten carbon atoms,or on styrene or on acryinitrile, and homo- or copolymers of cyclicolefins.

The following plastics may be mentioned as representatives of the abovegroups:

Polyacrylates having identical or different alkyl radicals having from 4to 8 carbon atoms, bonded to the oxygen atom of the ester group, inparticular having the n-butyl, n-hexyl, n-octyl, or 2-ethylhexylradical, polymethacrylate, polymethyl methacrylate, methylacrylate-butyl acrylate copolymers, methyl methacrylate-butylmethacrylate copolymers, ethylene-vinyl acetate copolymers, chlorinatedpolyethylene, nitrile rubber, acrylnitrile-butadiene-styrene copolymers,ethylene-propylene copolymers, ethylene-propylene-diene copolymers,styrene-acrylnitrile copolymers, acryinitrile-butadiene rubber,styrene-butadiene elastomers, and methyl methacrylate-styrene-butadienecopolymers.

The alicyclic polycarboxylic esters of the invention may moreover beused to modify plastics mixtures, for example the mixture of apolyolefin with a polyamide.

Mixtures made from plastics and the alicyclic polycarboxylic esters ofthe invention or the polycarboxylic esters prepared according to theinvention are also provided by the present invention. Suitable plasticsare the abovementioned compounds. Mixtures of this type preferablycomprise at least 5% by weight, particularly preferably from 20 to 80%by weight, very particularly preferably from 30 to 70% by weight, of thealicyclic polycarboxylic esters.

Mixtures made from plastics, in particular PVC, and comprising one ormore of the alicyclic polycarboxylic esters of the invention may bepresent in the following products, for example:

casings for electrical devices, such as kitchen machines, computercases, casings and components of phonographic and television equipment,of piping, of apparatus, of cables, of wire sheathing, of insulatingtapes, of window profiles, in interior decoration, in vehicleconstruction and furniture construction, plastosols, in floor coverings,medical products, packaging for food or drink, gaskets, films, compositefilms, phonographic disks, synthetic leather, toys, containers forpackaging, adhesive-tape films, clothing, coatings, and fibers forfabrics.

Mixtures made from plastic, in particular PVC, and comprising one ormore of the alicyclic polycarboxylic esters of the invention maymoreover be used for producing the following products, for example:

a casing for electrical devices, piping, apparatus, a cable, wiresheathing, a window profile, a floor covering, a medical product, a toy,packaging for food or drink, a gasket, a film, a composite film, aphonographic disk, synthetic leather, a container for packaging, anadhesive-tape film, clothing, a coating, or a fiber for fabrics.

Besides the abovementioned applications, the alicyclic polycarboxylicesters of the invention may be used as a component in lubricating oil,as a constituent of coolants, or as metal working liquids.

The examples below are intended to illustrate the invention withoutlimiting its scope of protection, as given in the description and theclaims.

EXAMPLE 1 Hydrogenation of Diisononyl Phthalate (Vestinol 9) ComparativeExample

590 g of Vestinol 9 are hydrogenated using pure hydrogen in the presenceof the ruthenium catalyst B4168/10r in a 600 ml pressure reactor at apressure of 200 bar and at a temperature of 120° C. Once hydrogen uptakehas ended, the reactor was depressurized and the product was analyzed.Diisononyl phthalate conversion was then 99.9%. The yield ofdi(iosononyl) cyclohexane-1,2-dicarboxylate was 99.8%, and a cis/transratio determined by means of ¹H NMR spectroscopy was 97:3.

Measurement device: Avance DPX-360 NMR spectrometer from the companyBruker Measurement frequency: 360 MHz Sample head: QNP sample head, 5 mmSolvent: CDCl₃ (degree of deuteration 99.8%) Tetramethylsilane (TMS)Standard: 303 K Measurement temperature: 32 Number of scans: 1 s Delay:4.4 s Acquisition time: 7440.5 Hz Spectral width: 30° Pulse angle: 3.2μs. Pulse length:

An example of the method of recording the ¹H NMR spectra compriseddissolving about 20 mg of the sample in about 0.6 ml of CDCl₃ (with 1%weight of TMS). The spectra were recorded under the conditions givenabove and referenced to TMS=0 ppm.

In the ¹H NMR spectra obtained, the methyne signals for cis- andtrans-dialkyl hexahydrophthalates could be distinguished with chemicalshifts of about 2.8 ppm and 2.6 ppm, respectively. The signal shiftedtoward lower field corresponding to the cis compound (larger ppm value).To quantify the isomer, the integrals were determined from 3.0 ppm to2.8(2) ppm and from 2.7(2) ppm to 2.5 ppm, the two integrals beingseparated in the middle between the signals. The ratio of the twoisomeric structures could be determined from the intensity ratios.

EXAMPLE 2 Synthesis of Monoisononyl Phthalate

444 g of phthalate anhydride (3 mol) and 432 g of isononanol (3 mol)(precursor of Vestinol 9) were heated slowly in a round-bottomed flaskwhich had internal thermometer and stirrer and on which a refluxcondenser had been placed. Monoester formation started at a temperatureof 117° C. and was discernible via a marked rise in temperature.Immediately after the temperature rise, the supply of heat wasinterrupted. After about 10 minutes the mixture, which had now reachedits final temperature of about 150° C., was cooled. The hydrogenationproceeds quantitatively.

Gas chromatography permitted a composition to be determined at about 95%by weight of monoester, 3% by weight of diester, 0.5% by weight ofisononanol and 1.5% by weight of phthalic acid.

EXAMPLE 3 Hydrogenation of Monoisononyl Phthalate

487 g (1.67 mol) of the monoester mixture prepared in example 2 weremixed, without further work-up, with 240 g (1.67 mol) of isononanol(precursor of Vestinol 9), and charged to a 1000 ml reactor undernitrogen. 74 g of the ruthenium catalyst B4168/10r were added andhydrogenation with hydrogen was carried out at 200 bar and 120° C. Oncethe ring-hydrogenation of the aromatic carboxylic derivatives had ended,the reactor was depressurized.

EXAMPLE 4 Esterification of the Ring-Hydrogenated Monoester to Give theDiester

The reactor discharge from example 3 was transferred to a standardesterification apparatus and mixed with a further 120 g (0.83 mol) ofisononanol and about 0.07 g of tetrabutyl titanate. Toluene was used asentrainer, and its amount in the reaction mixture was effective inkeeping the esterification temperature constant at 180° C. Theesterification proceeded with reflux of the alcohol and toluene chargeat atmospheric pressure, and the water produced was taken off at thewater separator. Esterification was carried out as far as an acid valueof <0.5 mg KOH/g. The alcohol was distilled off at a Claisen bridge atthis temperature until the final pressure of 10 mbar had been reached.The mixture was then cooled to 80° C. and neutralized dropwise with 10%strength aqueous sodium hydroxide solution, and subsequently stirred forapproximately 30 further minutes.

The ester was then heated to 180° C. in a 10 mbar vacuum. Distilled ordemineralized water (8%, based on the initial weight of crude ester) wasthen added dropwise at constant temperature via the immersion tube. Oncethe work-up had ended, the heating was switched off. The product thencooled in vacuo to 80° C. and was then filtered through a suction-typefilter funnel, using filter paper and filtration aid, to give a clearfiltrate.

The ratio of cis- and trans-cyclohexane-1,2-dicarboxylic diesters wasdetermined by ¹H NMR spectroscopy (see above). It was 51 to 49.

EXAMPLE 5 Esterification of the Ring-Hydrogenated Monoester to Give theDiester

The esterification of the product discharged from the reaction inexample 3 was carried out in a manner similar to example 4, except thatno toluene was added. This resulted in a rise in the esterificationtemperature to 250° C.

After work-up, investigation of the product by ¹H NMR spectroscopy gavea cis/trans ratio of about 21/79.

Examples 4 and 5 show that products with different trans contents forthe cyclohexane-1,2-dicarboxylic esters can be prepared by varying theesterification conditions.

The process of the invention results in higher trans contents than thehydrogenation of Vestinol 9 (comparative example 1), using the samehydrogenation conditions during the hydrogenation of the monoester andof the diester.

1. A process for preparing cycloaliphatic polycarboxylic esters, whichcomprises: a) hydrogenating a partial ester of the correspondingaromatic polycarboxylic acid or of the corresponding aromaticpolycarboxylic anhydride whose carboxylic acid or anhydride groups havebeen partially esterified with at least one branched or unbranchedalkyl, cycloalkyl, and/or alkoxyalkyl alcohol having from 3 to 15 carbonatoms, and b) reacting the resultant cycloaliphatic partial ester withat least one branched or unbranched alkyl, cycloalkyl, and/oralkoxyalkyl alcohol having from 3 to 15 carbon atoms in the presence ofa titanium or zirconium catalyst to give a full ester product.
 2. Theprocess as claimed in claim 1, wherein the hydrogenation of the aromaticpartial ester is carried out with a catalyst comprising at least onemetal of the 8^(th) transition group of the periodic table.
 3. Theprocess as claimed in claim 1, wherein the alcohol or alcohol mixturereacted with the cycloaliphatic partial ester is the same as that whichwould be obtained by saponifying the partial ester of the aromaticpolycarboxylic acid.
 4. The process as claimed in claim 1, wherein thealcohol or alcohol mixture reacted with the cycloaliphatic partial esteris other than that which would be obtained by saponifying the partialester of the aromatic polycarboxylic acid.
 5. The process as claimed inclaim 1, wherein the aromatic polycarboxylic acid or the correspondingpolycarboxylic anhydride comprises 2, 3 or 4 carboxyl functional groups.6. The process as claimed in claim 1, wherein the aromaticpolycarboxylic acid or the corresponding polycarboxylic anhydride iscomprised of one benzene ring or from 2 to 5 condensed benzene rings. 7.The process as claimed in claim 1, wherein the alcohol comprisesbranched or unbranched alkyl, cycloalkyl, and/or alkoxyalkyl groupshaving from 8 to 13 carbon atoms.
 8. The process as claimed in claim 1,wherein the partial ester is a partial ester of an aromaticpolycarboxylic acid and comprises a monoester of a benzene-1,2-, 1,3-,or 1,4-dicarboxylic acid.
 9. The process as claimed in claim 1, whereinthe partial ester is a partial ester of an aromatic polycarboxylic acidand is a mono- or diester of 1,2,3-, 1,2-4- or 1,3,5-trimellitic acid.10. The process as claimed in claim 2, wherein the alcohol or alcoholmixture reacted with the cycloaliphatic partial ester is the same asthat which would be obtained by saponifying the partial ester of thearomatic polycarboxylic acid.
 11. The process as claimed in claim 2,wherein the alcohol or alcohol mixture reacted with the cycloaliphaticpartial ester is other than that which would be obtained by saponifyingthe partial ester of the aromatic polycarboxylic acid.
 12. The processas claimed in claim 1, wherein the alcohol component of thecycloaliphatic partial ester is a C₈ or C₉ aliphatic alcohol or isomermixture of these alcohols and alcohol reactant of step (b) is a C₈ or C₉aliphatic alcohol or isomer mixture of these alcohols.